When fabricating the pole pieces of thin film magnetic recording heads or other high tolerance elements, new techniques have developed in electroplating and etching sheet materials whose composition and structure must be accurately controlled in order to achieve uniformity of performance. As taught in U.S. Pat. No. 3,853,715, conventional masking techniques are quite ineffective in producing these elements.
What has been done in the prior art, in dealing with the plating of an alloy, such as Permalloy, a mixture of nickel and iron, was to plate the alloy in sheet form and then etch the sheet into desired patterns. However, when depositing films by electroplating, it is necessary to employ an adhesive layer between the alloy and the substrate that will support the alloy pattern. Since on certain adhesive layers it is not possible to electroplate, it is at times necessary to deposit a thin layer of fairly noble metal, such as Au, Pt, Pd, Cu, Ni, etc. on the adhesion layer.
Unfortunately, many adhesive and plating base layers that are compatible with the magnetic alloy in the substrate become cathodic to the alloy during etching, producing severe undercutting. For example, nickel-iron alloy is made adhesive to glass or silicon by interposing a thin layer of chromium or titanium between the nickel iron alloy and its associated substrate. As taught in U.S. Pat. No. 3,853,715, when such plural layers are etched, a severe undercut is observed in the etched material. Such undercutting is due to several separate effects taking place during etching and is neither reproducible nor controllable. The undercutting is due to the fact that the chemical etching is an accelerated form of corrosion. The corrosion is isotropic in principle; it should take place at equal rates both normal to the thickness of the etched metal and parallel to the thickness. This results in uniform undercutting of the metal. But due to the extremely small film thicknesses and pattern dimensions, the dimensions of the metal crystallites and grain cannot be ignored. The grain boundaries and grains etch at different rates, resulting in ragged edges.
During the terminal stages of etching, when the adhesive and/or the plating base metal layers are exposed, the dissimilar metals form a galvanic cell which results in extremely rapid etching of the anodic metal. In case of titanium and chromium, each of these metals passivate extremely quickly and becomes cathodic to nickel, nickel-iron and to the metals of the iron group. When the metals such as platinum, palladium, gold or copper are present in the sandwich with the iron group metals, they act cathodically and the etching of the nickel, nickel-iron alloy, etc. is impossible to control.
Such undercutting is detrimental to the making of batch-fabricated arrays, such as the pole pieces of thin film magnetic heads.
U.S. Pat. No. 3,853,715 recognizes the above-described difficulties and teaches a solution. In order to achieve uniform etching of multi-layered electroplated metals without undercutting, the patent teaches placement of a very narrow border of photoresist on top of the cathodic adhesive metal layer prior to electroplating the anodic metal. The narrow border closes upon itself to serve as a frame, while a second photoresist layer is deposited and developed so as to be present only over the anodic material to be retained after etching. The second photoresist overlaps the first photoresist to completely encapsulate the anodic layer. It is taught that subsequent etching of the surplus anodic material not needed in the ultimate pattern leaves the desired portions of the pattern free from attack, avoiding the undercutting that occurs when two or more dissimilar metals are subject to a common etchant.
It has now been found that the prior art solution to the problem of undercutting is not entirely adequate. More specifically, it has been determined that after the electroplated anodic material, such as Ni--Fe alloy, has been etched away, the etchant then has access to the cathodic metal/adhesive layers and lateral etching occurs below the border photoresist. Once this begins to occur, the adhesive maintaining the photoresist in place loses its structural integrity and the photoresist begins to remove from the substrate further compounding the undercutting problem.